The present invention relates to fuel gas burners which have very low NO.sub.x emissions and in particular to such burners which operate with flue gas recirculated into the combustion air for the burner.
NO.sub.x is formed during the combustion of fuels, is discharged as part of the combustion gas, and constitutes a major atmospheric pollutant. Areas suffering high levels of air pollution, such as population centers in California, now have stringent pollution limits. For industrial burners, such as burners used in thermal power plants, NO.sub.x discharge limits can be as low as 9 ppm. Such low NO.sub.x levels in the flue gases are difficult to reach and require careful burner designs.
Industrial burners generate NO.sub.x in several ways. For example, industrial gas-fired burners generate NO.sub.x from oxidation of atmospheric nitrogen.
One formation mechanism, known as thermal NO.sub.x, generates NO.sub.x at a rate exponentially related to the peak temperature and proportional to the residence time of moles of gas at such temperature inside the flame. The production of thermal NO.sub.x can be effectively reduced by diluting combustion air with mostly inert gaseous products of combustion in a process known as flue gas recirculation (FGR). In some burners that generate premixed, or premixed like combustion, the generation of thermal NO.sub.x can be reduced, in a manner similar to FGR, by increasing the relative amount of combustion air.
Another mechanism of NO.sub.x formation, known as "prompt NO.sub.x ", generates NO.sub.x during the oxidation of molecules of fuel almost instantly as compared to the formation of thermal NO.sub.x. The production of prompt NO.sub.x is a strong function of local stoichiometric conditions at the moment of fuel oxidation. Pockets of fuel rich combustion are known to generate most of the prompt NO.sub.x, while less than 2 ppm of prompt NO.sub.x is formed when the local stoichiometric excess air is more than 10-20%.
Thus a combustion device in which the fuel is uniformly mixed with air in an amount slightly above stoichiometric and with a substantial amount of FGR generates very low NO.sub.x emissions because both thermal and prompt NO.sub.x are greatly reduced. Experiments show that 9 ppm NO.sub.x emissions with ambient combustion air in an amount 15% above stoichiometric requires about 30% FGR. With the FGR levels of 35-40%, NO.sub.x emissions can be reduced to below 5 ppm.
There are, however, several obstacles to making such a device suitable for a typical industrial application. The first is a reduced flammability of a mixture with such a large amount of FGR and the ensuing difficulty of stabilizing the flame. The second is the proneness of premixed flames to generate destructive pulsations in the combustion volume. The third is reduced turn-down capabilities. For a burner with a conventional nozzle, a turn-down ratio of 10:1 is typically easily achieved. Burners designed for premixed-type combustion, however, generally have a turn-down ratio of not more than about 5:1. Further, with premixed-type combustion burners it is difficult to achieve a uniformly mixed flow of air and fuel prior to fuel ignition without creating a significant volume of a potentially explosive mixture.
One of the known devices which achieves operation with NO.sub.x emissions below 9 ppm uses a very large number, e.g. 1000-1500, of spaced-apart fuel gas discharge openings--orifices. These openings are formed in numerous hollow vanes that traverse the entire flow of combustion air mixed with FGR, thus making the burner very costly.
Since there are so many fuel gas discharge orifices, their diameters must be very small, say on the order of about a few hundredths of an inch.
Such small orifices are easily plugged by even very small particles which may be present in the fuel gas. To prevent the clogging of the orifices, which in turn would upset the relatively even distribution of fuel gas injected into the air stream, the fuel gas requires filtering before it is introduced into the burner. In addition, to prevent corrosion of fuel gas conduits and particle shedding into the stream of fuel, more expensive materials, like stainless steel, must usually be selected for the fuel-carrying components of the burner, which further increases burner costs.
Although burners of the type described in the preceding paragraph can reach NO.sub.x emissions below 9 ppm, they tend to become unstable when operating at partial capacity. As a result, the margins of the operating parameters, within the stability limits of the burner, become more narrow at reduced loads. This limits the practical turn-down ratio for such burners to typically 4:1, even if coupled with the most accurate controls.
Thus, there presently is a need for very low NO.sub.x emitting burners which are economical to build and which are capable of reliably operating within a wide turn-down ratio.